1. Field of the Invention
This invention relates to a torque sensor based on the principle of magnetostriction, and more particularly, to an improved magnetostrictive torque sensor which is simpler, more accurate, and more economical than state of the art sensors as well as more suitable to mass production and usage.
2. Description of Prior Art
Engineers and scientists have sought a simple, reliable, accurate means for measuring torque in rotating shafts for well over a century. Applications for such a torque measuring apparatus include diagnosis, prognosis, and load level monitoring of a vast number of different types of rotary drive mechanisms such as automotive, ship, and plane engines; motors and generators of all types; oil drilling rigs; rotating machining tools; all electric power steering; robotics; and much more.
Further, measurement of mechanical power produced by an engine (or used by a generator) cannot be made without knowing both torque and rotational speed of the shaft. Hence there has heretofore been no ready means to determine on-line power and efficiency of rotary drive devices simply, accurately, and reliably. This has proven to be problematic in many areas of modern technology, but it has been particularly troublesome in attempts to develop modern automotive engine control systems which would improve fuel efficiency and optimize engine performance.
Heretofore several methods have been developed for measuring torque in rotating shafts (see below), but none has been ideal. That is, no single presently known method offers all of the following desirable properties.
1. Contact free (no slip rings, etc.) PA1 2. Reliable (low failure rate) PA1 3. Accurate PA1 4. Small and unobtrusive (requiring little shaft/engine re-work) PA1 5. Inexpensive PA1 6. Applicable at high as well as low speeds PA1 7. Instantaneous torque measurement (not merely mean torque over several revolutions) PA1 8. Amenable to mass production (not restricted to special test apparatus) PA1 1. Twist angle of shaft measurement PA1 2. Strain gauge sensor PA1 3. Reaction force measurement PA1 4. Magnetostrictive sensors PA1 (1) Non-contact: no slip rings PA1 (2) Not restricted to low speeds PA1 (3) Measures torque of engine directly PA1 (4) High sensitivity PA1 (5) Economical PA1 (6) Simple structure: no strain gauges, no large apparata PA1 (7) Only one location anywhere on shaft: little engine rework PA1 (8) Durable and reliable: no moving parts to cause mechanical failure, resistant to high pressure and temperature of engine environment PA1 (9) Readily miniaturized: can be made unobtrusive PA1 (1) Output signal varies with RPM even at constant torque. PA1 (2) Output signal varies with temperature. PA1 (3) Spurious signal variation within one mechanical cycle (one revolution of shaft) prohibits accurate instantaneous measurement of torque: only average values over several shaft revolutions are possible. PA1 (4) Correction methodologies such as those described in U.S. Pat. Nos. 4,589,290 and 4,697,459 and SAE paper #870472 heretofore employed for problems (1) to (3) above have not been able to reduce inaccuracies to an acceptable level. PA1 (5) All such correction methodologies developed to date involve complicated and extensive electronic circuitry and/or additional sensors for temperature and RPM. PA1 (6) Additionally, all such correction methodologies utilized to date are affected by subtle individual shaft material and property variations such as residual stress, slight inhomogeneity in shaft magnetic properties, shaft tolerances/misalignment, and shaft bending stress. Hence such methodologies must be tailored specifically for each individual shaft and therefore are not suitable for mass production. PA1 (7) Furthermore, such correction methodologies have to be re-calibrated repeatedly over the lifetime of the shaft, since residual stress values, tolerances, misalignments, bending stresses, and even magnetic property inhomogeneities change over time (particularly in high temperature environments such as those of automobile engines). Recalibration for mechanisms such as automobile engines is so difficult as to render such correction methodologies impractical. PA1 (1) Elimination of signal dependence on shaft rotation speed. PA1 (2) Elimination of signal dependence on temperature. PA1 (3) Elimination of signal variations within a single shaft revolution due to shaft magnetic property inhomogeneities and residual stresses. PA1 (4) Elimination of signal variations due to bending stress, shaft misalignment, and tolerance variations. PA1 (5) Instantaneous measurement of torque resulting from advantages (3) and (4) above. PA1 (6) Elimination of signal dependence on individual shaft properties and hence suitability for mass production. PA1 (7) Elimination of need to recalibrate sensing device during course of shaft lifetime. PA1 (8) Simple, effective signal processing circuitry and sensor orientation permitting advantages (1) through (7) above.
There are presently only four distinct methods for measuring torque directly in a rotating shaft. They are:
The twist angle method involves measurement of the angle of twist of a shaft and correlates this, using the material and dimensional characteristics of the shaft, to torque. It entails a complicated and cumbersome mechanism with low sensitivity, calibration difficulties, and the necessity of using two different locations along the shaft. It invariably entails extensive engine modification, a costly endeavor. The strain gauge approach requires bonding of strain gauges to the shaft surface and relating strain measurement to torque. It is limited to low speed, is not amenable to mass production, lacks durability, and needs some means such as slip rings and brushes to bring the signal off of the shaft.
Reaction force measurement utilizes Newton's second law for rotational motion to relate force and motion of the engine mounts to shaft torque. The method must employ a large structure, has low sensitivity, is not feasible for production runs, and measures driveline, not engine, torque.
Magnetostrictive torque sensors take advantage of the magnetostrictive property of ferromagnetic materials whereby tension stress increases (and compressive stress decreases) a given magnetic induction field (i.e., the "B" field) carried by the material. A coil of wire of arbitrary number of turns wrapped around an iron core is placed close to the shaft and an electric current passing through the wire causes a magnetic field to be induced in the rotating shaft. In magnetostrictive sensor designs such as those described in U.S. Pat. Nos. 2,912,642 and 4,589,290 a second coil of arbitrary number of turns wrapped around a second iron core is then placed close to the shaft and used to measure the change in the induction (the B field) which results from the increased surface stress caused by the applied torque.
The magnetostrictive method has several advantages over the other three methods, including
However, magnetostrictive torque sensors have heretofore been plagued with several major problems which have prevented them from becoming the standard in the field. These are